This invention relates to equalizer circuitry, and more particularly to equalizer circuitry that can be constructed to have selectably different operating characteristics for meeting a variety of different possible needs when the circuitry is put to use.
In general, the terms “equalizer,” “equalization,” or the like can be applied to either circuitry that transmits a signal (e.g., an electrical signal), or to circuitry that receives a signal. The present invention relates to equalizers in transmitter circuitry, and so it will be understood that all uses of terms like “equalizer” herein refer to circuitry for transmitting a signal or for preparing a signal for transmission. Another term that is sometimes used for equalization of this type (i.e., in transmitter circuitry) is pre-emphasis.
A typical reason for giving a signal that is to be transmitted equalization or pre-emphasis prior to transmitting the signal is to compensate for certain kinds of losses that the signal will experience as it travels through a transmission medium or channel to the circuitry that must receive and correctly interpret it. For example, it may be desirable to give a digital data signal extra (pre-emphasis) energy immediately following each transition in the level of the signal because a transmission channel can particularly degrade the “sharpness” or “crispness” of such transitions in conveying the signal to the receiver circuitry. Anticipating such degradation of transitions by giving them extra energy at the transmitter (i.e., by means of equalization or pre-emphasis) can improve the “quality” of the transitions as seen at the receiver, even after the inevitable degradation produced by the transmission channel. This can help the receiver circuit to more quickly and accurately detect the logical level (and changes or transitions in the logical level) of the received data signal. Such improvements to data reception can make possible the use of higher data transmission speeds without adversely affecting data transmission reliability.
Some devices (i.e., integrated circuit devices) are intended for relatively general purpose use. In other words, the device may be manufactured with the intention that it can be put to any of a variety of different uses. This may mean, for example, that the exact properties of a transmission channel (e.g., from that device to another device on a printed circuit board) that the device may be called upon to drive with an output signal may not be known to the device manufacturer. On the other hand, it is desirable for the manufacturer to be able to provide one device construction that can serve a variety of transmission channel characteristics, because that helps the manufacturer increase production volume of each of a smaller number of different device designs, thereby tending to decrease the unit cost of each device. Preferably, the device is manufactured so that each user can easily customize it to meet that user's particular needs (in this case, with respect to the operating characteristics of equalizer circuitry on the device).
Examples of the types of customizable devices that are referred to above are programmable logic devices (“PLDs”), field-programmable gate arrays (“FPGA”), programmable micro-controllers, and the like.
Heretofore, some devices of the types referred to above have included pre-emphasis or equalizer circuitry having some user-selectable operating characteristics. However, these user-selectable aspects have been only of limited kinds.